pass null parameter if Selectparameter is null

<asp:SqlDataSource ID="SDStry" runat="server" CancelSelectOnNullParameter="false"

SELECTCommand="SELECT  * FROM tbFirma WHERE KisiFK=ISNULL(@KisiFK,KisiFK)">

<SelectParameters>

        <asp:Parameter Name="KisiFK" ConvertEmptyStringToNull="true" />

</SelectParameters>

</asp:SqlDataSource>